But even when Schengen doesn't apply, arrivals from within the EU do not have to clear customs. That is why UK airports like LHR and MAN have blue channels in baggage halls that allow you to bypass customs. Of course customs can still choose to stop you for security purposes, but in practice the monitoring seems to consist of a CCTV camera.
This is why EU airports issue baggage tags with green edging on them - allows customs officers to immediately identify bags from within the EU.